1. Field of the Invention
The present invention relates to an ignition coil for an internal combustion engine, and more specifically to a stick-type ignition coil directly mounted in an ignition plug hole.
2. Description of the Related Art
One type of conventional stick-type ignition coil for an internal combustion engine includes a center core part having an axially-disposed rod-like core body. A resin spool around which a primary coil and a secondary coil are wound is disposed around the center core part, and resin is filled within a coil housing to insulate the first coil from the second coil. The resin filled within the housing not only acts as an insulator but also prevents the coils from becoming loose, as the resin flows between adjacent wire rods of the coils before hardening.
Another type of conventional ignition coil includes a center core formed by disposing a permanent magnet having almost the same outer diameter as the core body outer diameter at both axial ends of the core body to increase the amount of voltage generated by the ignition coil. Alternatively, an ignition coil may include a rubber cushion member instead of the above-described permanent magnet to reduce axially-directed force acting on the core body due to different expansion coefficients of the respective members to prevent magnetostriction of the core body.
However, as shown for example in FIG. 6, because the core body and the permanent magnet or the cushion member are formed to have almost the same outer diameter, a bulge 82 is created at the interface 94 between the core body 93 and the permanent magnet 95 or the cushion member composing the center core part 92 unless the core body and the permanent magnet or the cushion member are co-axially assembled. Further, because cracks or voids in the insulation will occur where the center core part 92 expands and contacts, along with the resin insulating member and casing members having different expansion coefficients, due to temperature fluctuation, the center core part 92 is covered by a thermo-contractive tube 97 as a resin-made elastic cushion member, for example, to prevent the cracks from occurring.
However, because the thermo-contractive tube 97 also covers the outside of the bulge 82, the thermo-contractive tube 97 also causes a bulge 97a. Therefore, assembly time is increased because the spool tends to catch on the bulge 97a during assembly of the center core part 92 into the spool of the secondary coil. Further, it is difficult to assure co-axial alignment of the primary coil, the secondary coil and the center core part 92 because the center core part 92 tends to tilt within the spool. As a result, voltage generated by the secondary coil is decreased, and desired high voltage cannot be applied to the ignition plug.
Further, because the thermo-contractive tube 97 is inhibited from shrinking uniformly due to the deformation in the vicinity of the bulge 97a, the end 97b of the thermo-contractive tube 97 comes off as shown in FIG. 6, the center core part 92 covered by the thermo-contractive tube 97 becomes larger than its predetermined size and the center core part 92 cannot be assembled at the predetermined position within the secondary spool, and the thermo-contractive tube 97 is damaged by a bulge 93b of the center core 93 opposite the bulge 97a.